Optical fibers have been used for information transmission or the like, and medical equipment and semiconductor production lines, e.g., for excimer laser used for lithography for semiconductor production lines.
An optical fiber is made of silica glass or the like, with the core of high refractive index covered with clad of low refractive index, the former being doped with germanium, phosphorus or the like to increase refractive index while the latter with boron, fluorine or the like to decrease refractive index.
On the other hand, an excimer laser, e.g., ArF or KrF laser, emits high-energy ultraviolet ray of 193 or 248 nm. The high-energy ultraviolet ray, so-called deep ultraviolet ray of 200 to 300 nm and so-called vacuum ultraviolet ray of 200 nm or less cannot be transmitted, because they are absorbed by H2Oor O2 while traveling in air, and greatly lost. Therefore, the exposure device equipped with an excimer laser tends to be large-sized due to necessity for providing the light path kept vacuum or filled with an inert gas. Application of optical fiber has been in demand to reduce size of the exposure device equipped with an excimer laser, because it allows the exposure device to be handled more simply.
Those devices which utilize deep ultraviolet or vacuum ultraviolet ray include excimer lamps, e.g., Xe2, KrCl and XeCl lamps which emit deep or vacuum ultraviolet ray of 172, 222 and 308 nm. These excimer lamps are used for surface cleaning devices by which contaminants attaching to semiconductor wafer and liquid-crystal display glass surfaces are optically decomposed by ultraviolet ray to be removed. Application of optical fiber has been also in demand by these surface cleaning devices equipped with an excimer lamp to reduce their size and facilitate their handling, as is the case with the exposure devices. Transmittance of an optical fiber changes with wavelength of the deep or vacuum ultraviolet ray to be transmitted, as shown in FIG. 13.
However, the conventional optical fiber is deteriorated when irradiated with ultraviolet ray. Referring to FIG. 14, deterioration of the optical fiber by the transmission of ultraviolet ray is represented by the phenomenon of decreased transmittance T6 with time, when a 1 m long silica glass having a core of 200 μm in diameter is irradiated with light emitted from a deuterium lamp (wavelength: 214 nm). Therefore, an optical fiber is treated with hydrogen to prevent deterioration of the transmission characteristics. Nevertheless, deterioration represented by that of transmittance T5 is unavoidable. Therefore, the conventional optical fiber is inapplicable to transmission of ultraviolet ray, because of its rapid deterioration by the transmission of ultraviolet ray.
On the other hand, recently, the so-called scanning near field optical microscope has been developed to detect near field light. It has been used for, e.g., observation or destruction of cells, DNAs and other very small objects.
The sample to be observed by this type of microscope is placed on a total reflection prism of inverted triangle shape, where the sample is irradiated with light from the back side under the conditions to satisfy the total reflection at the sample surface. This generates the surface wave referred to as near field light in the vicinity of the sample surface. When a probe with sharpened end, or sharpened probe, is inserted in the surface wave, the near field light is scattered, and part of the scattered light penetrates into the probe and is guided to the detection member, to measure optical information from the sample at a resolution of the order of nanometer.
The probe for a scanning near field microscope working on the above principle has been an optical fiber probe, with core of silica glass containing a given quantity of germanium, covered with a clad of silica glass.
However, the optical fiber probe of the above structure is difficult to propagate the so-called vacuum ultraviolet ray having a wavelength of 200 nm or less and so-called deep ultraviolet ray having a wavelength of 200 to 300 nm, which is one of its major disadvantages, because it is greatly deteriorated by the transmission of the vacuum or deep ultraviolet ray and is not fit for such ray.
Incidentally, for an optical fiber to be used for the probe for a scanning near field optical microscope, it is necessary to sharpen its end to a light wavelength or less. Its end can be sharpened by immersing it in an etchant solution, as disclosed by, for example, Japanese Patent Laid-open No. 104244/1998. How fast it is dissolved in the etchant solution depends on the etchant solution composition and material of each layer that constitutes the optical fiber.
However, the conventional etching method has a disadvantage that it cannot efficiently etch the optical fiber, because it is composed of the core of silica glass covered with clad, and is difficult to have a desired shape of the sharpened section at the fiber end.
The present invention is developed to solve these problems. It is the first object of the invention to provide an optical fiber for transmitting ultraviolet ray which has a high transmittance for deep ultraviolet ray, vacuum ultraviolet ray or the like, and is deteriorated only to a limited extent when irradiated with ultraviolet ray, and method of producing the same. It is the second object of the invention to provide an optical fiber probe which can propagate vacuum ultraviolet ray and deep ultraviolet ray at a high transmittance, is deteriorated only to a limited extent when irradiated with ultraviolet ray and can be etched to have a desired shape of the sharpened section at the optical fiber end, and method of producing the same.